The invention relates to a dynamic focusing circuit as defined in the precharacterizing part of claim 1. The invention also relates to a display apparatus comprising such a dynamic focusing circuit.
Such a dynamic focusing circuit is especially suitable for supplying a parabolically shaped dynamic focusing voltage with a high repetition rate to a focus electrode of a picture tube. Parabolically shaped should be understood to include any waveform having a falling edge, a somewhat flat part and a rising edge, successively.
U.S. Pat. No. 5,036,259 describes a dynamic focusing system for a cathode ray tube (CRT). The dynamic focusing system generates a parabola like dynamic focusing voltage which has a falling edge, a more or less flat part and a rising edge. A first npn transistor has a collector connected to a high supply voltage, an emitter connected to a focus electrode of the CRT via a capacitor, and a base connected via a series arrangement of two resistors to the high supply voltage. A bootstrap capacitor is arranged between the emitter of the first transistor and the junction of the two resistors. A second transistor has a collector connected to the base of the first transistor to draw a current away from the base of the first transistor in response to a waveform supplied between the base and the emitter of the second transistor. A diode has an anode connected to the emitter of the first transistor and a cathode connected to the base of the first transistor. During the flat part of the focusing voltage, the two resistors supply a bias current to the base of the first transistor. The bias current originates from the high supply voltage. The bootstrap capacitor is charged to a predetermined voltage. During the falling edge of the focusing voltage, the second transistor withdraws the bias current supplied by the resistors from the base of the first transistor, and the diode starts conducting thereby lowering the focusing voltage. When the diode is conducting, the first transistor inherently is non-conducting. During the rising edge of the focusing voltage the second transistor withdraws less current from the base of the first transistor. The bias current supplied via the resistors will flow into the base of the first transistor which starts conducting. The focusing voltage starts to rise. The voltage across the bootstrap capacitor initially stays the same. Consequently, the bias current supplied via the resistor between the capacitor and the base of the first transistor has the same value although the voltage across the series arrangement of the resistors decreases substantially. The dissipation in the resistors has been lowered significantly by using the bootstrap capacitor. In the prior art, the values of the resistors need to be selected so low that the required base current at a high value of the focusing voltage can be supplied. Such low values of the resistors result in a very high current and thus dissipation in the resistors at low focusing voltages as occur during the flat part of the focusing voltage.
It is a drawback of the prior art focusing circuit that the first transistor is non-conducting at the beginning of the rising edge of the focusing voltage. This causes a delayed rise of the focusing voltage and thus a non-optimal focusing of the electron beam on the screen of the CRT. The delayed rise of the focusing voltage can be minimized by enlarging the bias current, but this causes an unwanted increase of the dissipation in the series arrangement of the resistors. It is a further drawback of the prior art focusing circuit that, although the dissipation in the series arranged resistors has been lowered considerably by adding a bootstrap capacitor, it is still high.